Conventionally, wire channels have been formed in a semiconductor wafer by first depositing a conducting layer on the wafer surface using sputtering or a similar technique. Next, the unnecessary portions of the conducting layer are removed through a chemical dry etching process with a pattern mask formed of resist or the like.
In conventional processes, aluminum (Al) or an aluminum alloy has been used to form the wire circuit. However, wiring has been made thinner to keep up with the increased complexity of semiconductor devices. The increasing current density generates increased thermal stress and higher temperatures. This causes stress-migration or electro-migration, which grow more remarkable as the layers of aluminum or the like are manufactured thinner and give rise to such disorders as wire breakage or short-circuiting.
To avoid an excessive generation of heat in the wiring, a metal having a higher conductivity such as copper is required to form the wiring. However, it is difficult to perform dry etching on copper or a copper alloy that has been deposited over the entire surface as in the process described above. An alternative process would be to first form channels for the wiring according to a predetermined pattern and then fill the channels with copper or a copper alloy. This method eliminates the process of removing unnecessary parts of the conductive layer by etching, requiring only that the surface of the wafer be polished to remove uneven areas. The method has the additional benefit of being able to form simultaneously multiple areas called plugs that connect the tops and bottoms of channels.
However, the shape of these wiring channels and plugs have a considerably high aspect ratio (the ratio of depth to width) as the width of the wiring gets smaller, making it difficult to fill the channels with an even layer of metal using sputtering deposition. The chemical vapor deposition method (CVD) has been used for depositing various materials, but it is difficult to prepare an appropriate gas material for copper or a copper alloy. Further, when using an organic material, carbon from the material becomes mixed in with the deposition layer and increases the resistance.
Therefore, a method was proposed for performing electroless or electrolytic plating by immersing a substrate into a plating solution. With this method, it is possible to fill wire channels having a high aspect ratio with a uniform layer of metal.
When performing an electrolytic plating process, for example, generally a plating solution having a composition including copper sulfate and sulfuric acid is used. If the solution has a low concentration of copper sulfate and a high concentration of sulfuric acid, it is known that the plating solution will have high conductivity and great polarization, thereby improving throwing power and coating uniformity. In contrast, if the plating solution has a high concentration of copper sulfate and a low concentration of sulfuric acid, it is known that through the work of an additive the solution will have good leveling ability, in other words, plating will grow from the bottom of the fine pits formed in the substrate surface.
For this reason, performing a plating process using a plating solution having a composition superior in throwing power and coating uniformity to fill copper in the fine pits of a substrate having a large aspect ratio, the leveling ability of the solution is poor. The inlets of the fine pits will be blocked first before the pits are filled, thereby tending to form voids in the pits. On the other hand, using a plating solution with a composition superior in leveling ability will be inferior in throwing power and coating uniformity, resulting in unplated areas on the walls and bottoms of the fine pits.
Generally in these plating processes, a copper seed layer is formed on the bottom surface and area surrounding the fine pits of the substrate. However, when performing electrolytic plating directly on a barrier layer, such as TiN or TaN, the sheet resistance of the barrier layer is much larger than the resistance of the copper sulfate plating solution. As a result, needle-shaped crystals are formed in plating processes using copper sulfate solution, resulting in a plating layer having loose adherence.
In addition a copper pyrophosphate plating solution is also widely used because of its close adhesion due to high polarization and layered deposition property. However, copper pyrophosphate plating solution has poor leveling ability. Hence, when filling fine pits with copper in a plating process using copper pyrophosphate plating solution, the inlets to the fine pits become blocked first, thereby developing voids, as described above. Of course, it is also possible to use copper pyrophosphate plating solution as a first layer over a copper seed layer.